Boson Peak and High Frequency Modes in Amorphous Silica

Low and high frequency modes in fused silica at normal and very high density are investigated by molecular dynamics simulation. The calculated vibrational density of states, the Raman intensity, and the far infrared spectrum of normal silica glass present a low frequency mode $\left(\sim 50{\mathrm{cm}}^{-1\mathrm{}}\right)$ whose shape and attenuation with temperature is closely related to the boson peak observed experimentally. This mode is attributed to concerted motions between Si${\mathrm{O}}_4$ tetrahedra. Finally, the present analysis supports the recent suggestion that silica glass becomes fragile at very high density.